In recent years, there has been extensive development of projection apparatus that include: a polarization conversion device; a light-splitting optical system; optical path reflecting means; image display elements; and a projection optical system. The polarization conversion device provides uniform polarization of light from a white light source. The white light source can be: a short-arc metal halide lamp, in which a metal halogenide is sealed in a light-generating tube, the distance between the electrodes is kept short, and the light-emission properties of the metal are used to generate light; a super high pressure mercury lamp, which can easily provide high luminance; a xenon lamp, which provides superior color rendering properties; or the like. The light-splitting optical system separates light into red (R), green (G), and blue (B) components. The red, green, and blue lights are each associated with optical path reflecting means and an image display element. Light from these image display elements are modulated using a video signal, and the resulting light is guided to a color combining optical system formed by combining prisms. This color combining optical system performs superimposition, and the resulting color image is magnified and projected onto a screen by the projection optical system.
These devices require high-precision control of light polarization properties. The polarization beam splitter, which acts as a polarizer and light detector, determines the polarization properties, and therefore greatly affects the image quality of the projected image. For this reason, much research has been done in recent years on improving polarization beam splitter properties.
As an example, a polarization beam splitter includes: a multilayer dielectric film; and a member formed from a translucent material. The absolute value of the photoelastic constant of the translucent member is selected so that it is no more than a predetermined value associated with the wavelength of the incident light projected into the translucent member. Also, the polarized light separation module associated with red (R), green (G), and blue (B) light beams separate the light from the white light source into their associated colors. The base materials used in the optical members of these polarized light separation modules are designed to provide optimal characteristics by providing the lowest absolute value of the photoelastic constant at different wavelengths.
These polarized beam splitters are used as polarizers or light detectors. Various improvements have been made on illumination optical systems to provide efficient and uniform distribution of light beams from the light source. In one implementation, a television image or an image output from a computer is displayed on image display elements arranged in a matrix according to a predetermined vertical/horizontal aspect ratio, e.g., a liquid crystal panel, serving as means for modulating the intensity of the illumination light beam described above. The image displayed by the image display elements is displayed on a projection-type image projector apparatus that uses a projection lens to magnify the image or a xe2x80x9crearxe2x80x9d type projection image display apparatus equipped with a screen on which to show magnified and projected images.
A screen aspect ratio of 4:3 (horizontal:vertical) is common for image display elements used to display computer screens. For displaying television images, the increase in digital cable broadcasts in the North American market and BS digital broadcasts in Japan has led to the growing popularity of an aspect ratio of 16:9 (horizontal:vertical) over the conventional 4:3 (horizontal:vertical) aspect ratio.
As described above, the absolute value of the photoelastic constant of the translucent member is selected so that it is no more than a predetermined value associated with the wavelength of the incident light projected into the translucent member. Also, the polarized light separation module associated with red (R), green (G), and blue (B) light beams separate the light from the white light source into their associated colors. The base materials used in the optical members of these polarized light separation modules are designed to provide optimal characteristics by providing the lowest absolute value of the photoelastic constant at different wavelengths. However, these methods are not adequate. Due to non-uniformity in the polarized light after the light passes through the polarizing beam splitter, the quenching ratio of the S-polarized light and the P-polarized light is reduced, leading to color and luminance unevenness in the magnified projected image on the screen.
While various methods have been used to provide optimal characteristics in the illumination optical system and the color combining system described above, the efficiency of the technology is still inadequate in practice.
Furthermore, no effective method for controlling the screen display positions of the image lights from the R, G, B image display elements has been established for implementations where the projection apparatus described above is installed in a xe2x80x9crearxe2x80x9d image display apparatus so that the magnified projection image is projected via a reflecting mirror onto a transmissive screen disposed at a predetermined position of a cabinet.
A first object of the present invention described above is to provide a projection apparatus with minimal unevenness of color and luminance in a magnified image projected to a screen. A second object is to provide an illumination optical system and a color combining optical system that can make efficient use of light beams generated from a light source. A third object is to provide optimal display positions for image display elements corresponding to red, green, and blue images, when the projection apparatus of the present invention is installed in a rear-projection image display apparatus.
[Means for Solving the Problems]
In order to overcome the problems described above, the present invention provides a projection apparatus including: polarized light converting means converting a light beam from a light source to a uniformly polarized wave; color separating means separating converted beam into a first, a second, and a third color beam based on wavelength ranges; first, second, and third optical path modifying means modifying optical paths associated with separated colors; first, second, and third polarized light separating means receiving color beams for which optical paths have been modified by optical path modifying means; first, second, and third reflective image display elements receiving first, second, and third color beams from polarized light separating means and outputting reflection beams from polarized light separating means; color combining means combining reflected beams from the reflective image display elements received via polarized light separating means; projecting means magnifying and projecting a color image combined by color combining means. First or second or third optical path modifying means is a reflective mirror with characteristics for transmitting long-wavelength light beams with wavelengths of at least 560 nm.
According to another aspect of the invention, color combining means combines reflected beams from the reflective image display elements received via polarized light separating means. Color combining means is formed by joining prisms on which are disposed optical filters selectively transmitting or reflecting light with predetermined wavelengths. The prisms are formed from at least two types of base materials having different wavelengths at which an absolute value of a photoelastic constant is lowest.
According to another aspect of the present invention, an an anti-reflection coating is disposed at an air boundary surface between polarized light converting means and the color combining optical system. The reflection prevention film is formed with a lowest reflectivity Rg and Rr, Rg, Rb satisfying the relationship 6 greater than Rr/Rg, 10 greater than Rb/Rg, where Rg is a reflectivity for light having a wavelength of approximately 550 nm, Rr is a reflectivity for light having a wavelength of approximately 630 nm, and Rb is a reflectivity for light having a wavelength of approximately 430 nm.
According to another aspect, the present invention includes: color separating means separating a light beam from a light source into a first, a second, and a third color beam based on wavelength ranges; image display elements receiving the first, the second, and the third color beams from color separating means, surplus image elements that can be activated being arranged outside of an effective display region; color combining means combining color beams from the image display elements; projecting means magnifying and projecting a color image combined by color combining means; a screen installed at a predetermined position and onto which projection image light from projecting means is projected; and at least two light-receiving elements detecting light intensity at the outside of the screen. Detection results from the light-receiving elements associated with the first, the second, and the third projection image lights are used to control an active region for the image display elements.